As the word itself implies, “cryoablation” involves the ablation of tissue (i.e. tissue necrosis or destruction) using extremely low (i.e. cryogenic) temperatures. Typically, cryoablation requires lowering the temperature of the tissue to below approximately minus twenty degrees Centigrade (−20° C.). However, more efficient ablation procedures often call for temperatures as low as minus eighty eight degrees Centigrade (−88° C.) or lower. At these low temperatures, portions of the tissue and surrounding body fluids (e.g. blood), which would otherwise be in a liquid state, freeze and become solid. The result is commonly referred to as an “ice ball.”
It is often desirable to cryoablate internal tissue in a relatively non-invasive procedure. For this purpose, cryocatheters have been developed, such as the cryocatheter and associated refrigeration system that is disclosed in co-pending U.S. patent application Ser. No. 10/243,997, entitled “A Refrigeration Source for a Cryoablation Catheter.” Co-pending U.S. application Ser. No. 10/243,997 was filed on Sep. 12, 2002, is assigned to the same assignee as the present invention, and is hereby incorporated by reference herein. In one exemplary application of a cryocatheter, conduction blocks can be created that are particularly effective for curing heart arrhythmias, such as atrial fibrillation.
In a typical cryocatheter procedure, the distal portion (i.e. cryotip) of the catheter is positioned near or in contact with the tissue requiring ablation (i.e. the target tissue). Next, the cryotip is cooled to a cryogenic temperature to thereby cool and ablate the target tissue. During cooling of the cryotip, an ice ball forms and grows. Eventually, the entire tip becomes covered with ice and the size of the ice ball stabilizes. In a typical procedure, the stable ice ball is maintained for a predetermined residence time (e.g. 5 minutes) to achieve an effective tissue ablation.
With the above in mind, it would be desirable to assess and monitor the formation of the ice ball for several reasons. For one, the formation of an ice ball provides an indication that the cryotip is correctly positioned relative to the tissue. In the case where the cryotip is improperly positioned (e.g. when the cryotip is still fully immersed in the bloodstream) an ice ball will not usually form. In addition, monitoring the time at which the size of the ice ball stabilizes facilitates the application of an accurate and consistent ice ball residence time. This results in an effective cryoablation with minimal complications.
In light of the above, it is an object of the present invention to provide systems and methods suitable for the purposes of assessing the formation of an ice ball during a cryoablation procedure. It is another object of the present invention to provide systems and methods for assessing the formation of an ice ball using measurement signals that do not adversely affect the electrical function of the heart. It is yet another object of the present invention to provide systems and methods for assessing an ice ball which are easy to use, relatively simple to implement, and comparatively cost effective.